1. Field of the Invention
The present invention generally concerns the manufacture and use of carbon electrodes and, more particularly, pertains to carbon anodes used for the production of aluminum in which the detrimental characteristics of dusting and gasification are reduced.
2. Description of the Prior Art
As is well known in the art, petroleum coke is used for the manufacture of carbon electrodes which are utilized in the aluminum industry. The description of such utilization for making aluminum can be found in Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 1, p. 941.
Prior to the formation of electrodes, the refinery petroleum coke must first be calcined. Calcination usually occurs with temperatures in excess of 2200.degree. F., and preferably above 2500.degree. F. The calcination densifies the coke and removes volatile matter therefrom, while changing the carbon to hydrogen ratio.
The coke exiting the calciner at these high temperatures must then be cooled. This is typically accomplished in a cooler wherein water is sprayed onto the coke. The water, by evaporation, cools the coke to a suitable temperature. This calcined, cooled coke is then formed into electrodes.
Carbon electrodes, or anodes, are typically formed from mixing regular grade coke, binder pitch, and sometimes butts (recycled anode remnants). The mixture is extruded under high pressure to form a green anode. The anode is subsequently baked in a furnace and placed in the electrolytic cell or pot-line in which aluminum is produced.
The normal aluminum pot reaction in an electrolytic cell is governed by the formula: EQU 2Al.sub.2 O.sub.3 (ore)+3C (electrode)=&gt;4Al (product)+3CO.sub.2
This reaction provides for a theoretical amount of consumed carbon to be 0.334 lbs. of carbon per lb. of produced aluminum.
A number of problems, associated with this reaction, are also well known in the aluminum industry. Actual carbon consumption in the above-described reaction is typically 0.45-0.50 lbs. of carbon per lb. of produced aluminum. It is believed that a major cause for this increased consumption, and consequent decrease in efficient operating life, is due to gasification and erosion of carbon dust particles at the anode in the electrolytic cell. The carbon dusting and gasification occur when carbon dioxide, evolved from the reduction process, passes over the anode and reacts with the carbon to form carbon monoxide, according to the formula: EQU CO.sub.2 +C=&gt;2CO
The carbon monoxide can subsequently decompose to form carbon dust and CO.sub.2. This formation of carbon dust, by definition, decreases the amount of carbon available to produce aluminum. The particles of carbon also float to the surface of the bath, causing additional problems within the cell. These carbon particles can provide a short circuit between the anode and cathode of the cell as well as form a heat insulating layer on the surface of the bath which can result in the overheating thereof.
As a consequence of the foregoing problems, tests have been developed to measure a carbon electrode's dusting and gasification characteristics within an electrolytic cell. Gasification is defined as the amount of gas released during the dusting of an electrode, i.e., residue wt.%+dusting wt.%+gasification wt.% equals one electrode. A related property of ground coke, which is well known in the art, is the coke's carboxy reactivity, which is defined as the ability of ground coke to react with CO.sub.2 to form carbon monoxide.
The chemical mechanism believed to be involved in promoting dusting and gasification of an electrode is the action of cations, especially Na, Mg, Ca and Fe, present as ash components which reside in or on the surface of the electrode, which catalyze the reaction of CO.sub.2 with the carbon in the electrode. These cations are also known to effect the carboxy reactivity of coke before it is formed into an electrode. As is shown in U.S. Pat. No. 4,341,751, it is known that the carboxy reactivity of calcined coke is reduced by providing the quench water with, preferably, orthophosphoric acid which acts to remove these cations from the surface of the coke. It has subsequently been found that, although this treatment of the quench water does indeed inhibit the carboxy reactivity of calcined coke, it does not significantly alter the dusting and gasification characteristics of the formed carbon electrode and, consequently, does not alleviate any of the aforementioned problems associated with dusting and gasification and the lowered production efficiency of the anode resulting therefrom.
Additionally, another problem exists in large electrode producing ovens. Typically, a number of anodes are stacked in an oven, and baked at an elevated temperature of approximately 2200.degree. F. for about 30 days to produce aluminum as is well known in the art. During the baking process, a temperature gradient develops across the bank of anodes within the oven. The outermost anodes vary in temperature by as much as .+-.200.degree. F. from the internally positioned anodes. It has been found that these outer anodes have increased dusting and gasification and, therefore, decompose at a much greater rate than the centrally located anodes. Accordingly, because this temperature gradient exists, there is a strong need to develop anodes having improved dusting and gasification characteristics which exist independently of temperature variations within the oven.